MIMO communication method, terminal, and base station apparatus for transmitting and receiving pilot signals to estimate a channel

ABSTRACT

A MIMO communication method of performing MIMO communication between a base station having a plurality of antennas and each of a plurality of terminals covered by the base station using uplink data slots and downlink data slots that are alternately placed on a time axis. The method includes, in the base station, despreading a received signal that is transmitted from each of the plurality of terminals demodulating the transmission data transmitted from a respective terminal on the basis of the value of the estimated channel; decoding a received signal included in the uplink data slots, estimating a current channel between each of all antennas of the base station and the respective terminal; and comparing the stored value of the estimated channel with a value of the estimated current channel and updating the stored value of the estimated channel to the value of the estimated current channel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims priority under 35 U.S.C.§119(e) to Provisional Application Ser. No. 62/008,898, filed Jun. 6,2014, the contents of which are hereby incorporated herein by reference.

BACKGROUND

1. Field

The present disclosure relates to MIMO (Multiple-Input, Multiple-Output)communication methods, and, more particularly, to a MIMO communicationmethod of transmitting and receiving a pilot signal.

2. Description of the Related Art

In current years, with the widespread use of smartphones, the amount ofwireless communication performed by mobile terminals has explosivelyincreased. It is expected that communication between devices withouthuman intervention will be performed in the future. From 2020 onward, atraffic volume is likely to increase by a factor of 1000 to 10000.Therefore, a new communication system is actively being studied. Thatis, in addition to LTE and LTE-A that have already been commerciallypractical, a system with which higher frequency utilization efficiencyis achieved is being studied and is expected to be proposed as the fifthgeneration system of the 3rd Generation Partnership Project (3GPP).

Massive MIMO is one of effective technologies.

As illustrated in FIG. 10, in a communication system, a base station BShas many (Nb) transmission antennas (Ant#1 to Ant#Nb) and communicateswith a plurality of (N) terminals (pieces of User Equipment: UE#1 toUE#N) at the same time via channels h00 to h0(N-1) to channels h(Nb-1) 0to h(Nb-1)(N-1). The number of transmission antennas (Nb) in the basestation BS is, for example, approximately 100, and the number ofterminals is, for example, approximately 10.

With this structure, frequency utilization efficiency that has beenaffected by communication path noise and rapidly changing fading can bestably obtained almost without such effects. As compared with MIMO inthe related art, a communication path capacity markedly increases. Fromthese reasons, the massive MIMO is the most promising next-generationtechnology.

The massive MIMO system was introduced in a paper around 2010 and hasbeen studied for its practical use. The results of the study have beenactively reported since the middle of 2013. Detailed examples of theresults of the study are, for example, Non-Patent Literatures 1 and 2.Referring to these literatures, the most important part of the study ischannel estimation that is performed with pilot signals. For example,when the number of antennas of a base station is 100 and the number ofterminals (each having one antenna) is 10, MIMO channels become a 10×100matrix. This state is quite different from that in a MIMO system in therelated art. More accurate channel estimation is required.

In a massive MIMO data section, terminals simultaneously performtransmission operations that overlap one another. The transmittedsignals are received by many receiving antennas of the base station BS.The autocorrelation of the received signals is performed for channeldiagonalization. The channel diagonalization allows the terminals to bedistinguished one another. When the channel diagonalization isincompletely performed, interference among the terminals occurs andcommunication fails. For accurate diagonalization, more accurate channelestimation is required.

A channel estimation method that is currently the mainstream isperformed with pilots that are transmitted from terminals and receivedby a base station. In this method, in order to avoid interference amongthe pilots transmitted from the terminals, it is assumed that the pilotsare transmitted in slots that do not overlap on a time-frequency axis.

This method will be described with reference to FIG. 9. In the presentdisclosure, as a method of distinguishing between an uplink(transmission from a terminal to a base station, Up Link: UL) and adownlink (transmission from the base station to the terminal, Down Link:DL), time division duplex (TDD) is employed. In the TDD method, theuplink and the downlink are alternately arranged on a time axis so as toprevent a situation in which different channels are used for the uplinkand the down link and to flexibly adapt to the change of a trafficvolume. For the simplification of the drawing, eight terminals areillustrated in FIG. 9.

A signal stream includes headers preceding data slots both in uplink anddownlink. When pilot signals reach a base station, a delay occursbecause of multipath. Assuming that the maximum delay time is Tg, thenumber of pilots that do not mutually interfere with one another in asingle TDD slot Ts is Ts/Tg. Referring to FIG. 9, four pilots areincluded in a single TDD slot Ts. Since all pilots for eight terminalscannot be included in this state, the transmission of pilots is alsoperformed with the next TDD slot. While a certain terminal transmits apilot, the other terminals cannot perform transmission. In the drawing,this term is illustrated as NA (Not Available). Thus, a pilot periodincreases a total overhead and degrades frequency utilization efficiencyand throughput.

A result of a single channel estimation is maintained on condition thata terminal is not moved. However, when the terminal moves and a channelvariation occurs, diagonalization cannot be performed. Accordingly, itis necessary to perform channel update at regular time intervals. Thistime interval is referred to as coherence time. The base station BSneeds to have more accurate channel information at all times. It istherefore desirable that the base station BS perform channel update inthe shortest possible time. On the other hand, however, the channelupdate performed on a terminal that is not being moved causes anincrease in overhead.

Furthermore, many control signals are transmitted between the basestation BS and the user equipment UE. Since the transmission of thesecontrol signals is also performed with headers, when headers areoccupied with pilots and control signals, overhead may increase and atotal throughput may decrease.

CITATION LIST Non Patent Literature

[NPL 1] Thomas L. Marzetta, “Noncooperative Cellular Wireless withUnlimited Numbers of Base Station Antennas”, IEEE TRANSACTIONS ONWIRELESS COMMUNICATIONS, VOL. 9, NO. 11, NOVEMBER 2010, pp. 3590-3600

[NPL 2] Fredrik Rusek, Daniel Persson, Buon Kiong Lau, Erik G. Larsson,Thomas L. Marzetta, Ove Edfors, and Fredrik Tufvesson, “Scaling upMIMO”, IEEE SIGNAL PROCESSING MAGAZINE, January 2013, pp. 40-60

In the case of the method in the related art, in addition to theabove-described problem that is the increase in overhead, anotherproblem is present. The problem is that a certain base station detects apilot transmitted from a neighboring cell to a cell of the base station,and is referred to as pilot contamination.

In the method in the related art, for different cells, the same pilottransmission slot and the same kind of pilot signal are used so as toset a repetition rate to “1”. This leads to the increase in frequencyutilization efficiency. When pilot contamination occurs, that is, a basestation receives a pilot from a terminal in another cell, the basestation misidentifies the terminal as if the terminal was in a cell ofthe base station. As a result, the terminal is accessed by a pluralityof base stations and interference occurs.

Furthermore, in massive MIMO, when a plurality of terminals are in closeproximity to each other, it is difficult to perform the separationbetween channels for the terminals. In this case, even if channelcorrelation is performed, the separation between signals cannot beachieved and communication fails.

An inventor recognizes the need for a pilot signal communication methodsuitable for massive MIMO.

BRIEF SUMMARY

According to the present disclosure, there is provided a MIMOcommunication method of performing MIMO communication between a basestation having a plurality of antennas and each of a plurality ofterminals covered by the base station using uplink data slots anddownlink data slots that are alternately placed on a time axis. The MIMOcommunication method includes: in each of the plurality of terminals,setting pilot transmission headers to be used for transmission of apilot signal at predetermined header intervals as headers of the uplinkdata slots; spreading the pilot signal in the pilot transmission headersusing corresponding one of orthogonal codes that are assigned to theplurality of terminals and are orthogonal to one another andtransmitting the spread pilot signal; spreading the pilot signal in theuplink data slots using a data orthogonal code that is different fromthe orthogonal code used in the pilot transmission headers,superimposing the spread pilot signal on transmission data, andtransmitting the transmission data; in the base station, despreading areceived signal that is transmitted from each of the plurality ofterminals and is included in the pilot transmission headers using theorthogonal code assigned to the terminal, estimating a channel betweeneach of all antennas of the base station and the terminal, and storing avalue of the estimated channel; demodulating the transmission datatransmitted from the terminal on the basis of the value of the estimatedchannel; decoding a received signal included in the uplink data slotsusing the data orthogonal code assigned to the terminal and estimating acurrent channel between each of all antennas of the base station and theterminal; and comparing the stored value of the estimated channel with avalue of the estimated current channel and updating the stored value ofthe estimated channel to the value of the estimated current channel whena difference between the values is larger than a value set in advance.

Since pilot transmission headers are placed at predetermined headerintervals, headers can be prevented from being occupied with pilots, aheader load is reduced, and the decrease in total throughput can beprevented.

Since a pilot signal is spread in uplink data slots using an orthogonalcode that is different from a code used in pilot transmission headers,is superimposed on transmission data, and is transmitted, a base stationcan perform channel estimation at the time of data reception and canalways obtain the latest channel information.

Using orthogonal codes, signals from terminals in close vicinity of oneanother can be separated. Furthermore, since signals from the othercells can be separated, pilot contamination can be solved. In thepresent disclosure, since a long-period orthogonal code can be used forthe superimposition of a pilot signal on data in data slots, a codeshortage does not occur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the time slot configuration of a signalstream transmitted/received from/by a base station in massive MIMOaccording to a first embodiment of the present disclosure.

FIG. 2 is a diagram illustrating the configuration of a transmissionunit in user equipment and the configurations of units for performingreceiving processing and data processing in a base station according tothe first embodiment.

FIG. 3 is a diagram illustrating a first modification of the firstembodiment.

FIG. 4 is a diagram illustrating a second modification of the firstembodiment.

FIG. 5 is a diagram illustrating the configuration of a transmissionunit in user equipment and the configurations of units for performingreceiving processing and data processing in a base station according toa second embodiment of the present disclosure.

FIG. 6 is a graph illustrating a result of characteristic simulationaccording to an embodiment of the present disclosure.

FIGS. 7A and 7B are diagrams illustrating a situation in which theconstellation of received signals rotated because of Doppler effect isrecovered after a channel estimation update has been performed with apilot superimposed on data.

FIG. 8 is a graph illustrating a simulation result of the effect of anamplitude adjustment factor Pg according to an embodiment of the presentdisclosure.

FIG. 9 is a diagram describing a known method of transmitting pilotsfrom terminals for the purpose of channel estimation.

FIG. 10 is a diagram illustrating an exemplary massive MIMOconfiguration.

DETAILED DESCRIPTION

The first embodiment of the present disclosure will be described indetail below.

FIG. 1 is a diagram illustrating the time slot configuration of a signalstream transmitted/received from/by a base station in massive MIMOaccording to an embodiment of the present disclosure.

In this embodiment, a single base station covers Nv mobile terminals(UE#1 to UE#Nv). In the following description, a mobile terminal ismerely referred to as a terminal. The distance between each terminal anda base station is measured in advance and a transmission time at theterminal is adjusted in accordance with the distance, so that signalstransmitted from the terminals can be synchronized at a receiving point.This method is used for Long Term Evolution (LTE). In a signal stream,uplinks (ULs) and downlinks (DLs) are time-separated through timedivision duplex (TDD). The same frequency is used for the ULs and thesame frequency is used for the DLs.

A header is set so that it precedes each of the uplinks and thedownlinks. There are two types of UL headers, a first UL header and asecond UL header. The first UL header is a header dedicated to pilottransmission (pilot header). The pilot headers are not all UL headersand are dispersedly located at regular intervals in a stream. The secondUL header is a general-purpose header used for the other purposes. Allof DL headers are general-purpose headers.

In this embodiment, a pilot signal of “1” is used. Pilot orthogonalcodes Pcode#1 to Pcode#Nv are assigned to a plurality of pieces of userequipment. Each piece of user equipment spreads the pilot signal of “1”using the assigned orthogonal code and transmits the spread pilot signalusing a pilot header. The “spread” means that exclusive OR processing isperformed with a pilot signal and an orthogonal code.

The base station BS receives pilots from all pieces of user equipment atthe same time, and can distinguish among these pieces of user equipmentby performing despreading using the orthogonal codes. The “despread”means that exclusive OR processing is performed with a received signaland the orthogonal code and then addition processing is performed overan overall code length. This process is equivalent to the inner productof vectors.

By using the pilot orthogonal code for the pilot signal received fromthe piece of user equipment, a channel H0 between each of all antennasof the base station BS and the piece of user equipment can be estimated.

Subsequently, each piece of user equipment transmits data using a ULdata slot. At that time, a pilot is spread with one of data orthogonalcodes Dcode#1 to Dcode#Nv that are different from the orthogonal codefor the pilot described above. A result of the spreading is multipliedby an amplitude adjustment factor Pg and is then multiplexed(superimposed) on data. That is, the piece of user equipment transmitsdata Tx_data obtained from the following equation (1) as data for a userequipment number jv.Tx_data=data+Pg·Dcode#jv·Pilot  (1)

The addition of the first term and the second term on the right side ofequation (1) is performed for each bit. That is, the length of the dataorthogonal code Dcode is in agreement with the total data length. As isapparent from equation (1), while a pilot signal is spread with anorthogonal code, data is merely superimposed.

The base station BS receives a signal Rx_data represented by thefollowing equation (2).

$\begin{matrix}\begin{matrix}{{Rx\_ data} = {H\; 0{r \cdot {Tx}}}} \\{= {H\; 0{r \cdot \left( {{data} + {{{Pg} \cdot {Dcode}}\#{{jv} \cdot {Pilot}}}} \right)}}}\end{matrix} & (2)\end{matrix}$

In this equation, H0 r represents a channel. The channel H0 r may bechanged from the initial channel H0 obtained from the pilot header.

The base station performs despreading on the received signal Rx_datausing the data orthogonal code assigned to the piece of user equipmentso as to obtain a channel-related receiving signal. This process isrepresented by the following equation (3).Channel-related receiving signal=Rx_data(x)Dcode#jv ^(T)=H0r·(data+Pg·Dcode#jv·Pilot)(x)Dcode#jv ^(T)  (3)

In this equation, an index T represents transposition and (x) representsa vector inner product operation (despreading). In equation (3), sincedata is random and its autocorrelation is low, the inner product of thedata and the long-period code Dcode is substantially zero. Consequently,when the length of the data orthogonal code Dcode is LN, equation (3)becomes the following equation.Channel-related receiving signal=H0r·Pg·LN·Pilot  (4)

From equation (4), the channel H0 r is obtained by Channel-relatedreceiving signal/(Pg·LN·Pilot).

Demodulated data is obtained from the following equation.(Rx_data/H0r)−Σ_(jv) Pg·Dcode#jv  (5)In equation (5), the second term of (Σ_(jv)Pg·Dcode#jv) is known and itsvalue is prepared in advance.

As is apparent from the following equation (6), the demodulated data canbe obtained by multiplying equation (5) by H0*. This multiplication isperformed for correlation.{(data+Pg·Dcode#jv·Pilot)−Σ_(jv) Pg·Dcode#jv}×H0*  (6)where “*” of H0* represents a complex conjugate transpose.

Next, operations will be described.

When a piece of user equipment is placed under the control of a basestation (is present in a corresponding cell) and is powered on, aregistration request is issued and initial setting is performed. At thattime, the base station assigns two types of orthogonal codes, the pilotorthogonal code and the data orthogonal code, to this piece of userequipment.

Each piece of user equipment spreads a pilot signal in a slot for apilot header using an assigned pilot orthogonal code and transmits thespread pilot signal. At the time of data transmission, each piece ofuser equipment multiplexes (superimposes) a data pilot on data andtransmits the data.

The base station first receives the pilot header and estimates thechannel H0 between each of all antennas of the base station and thepiece of user equipment using the pilot orthogonal code assigned to thepiece of user equipment. The channel H0 is stored in the base station.Subsequently, the base station receives from the piece of user equipmentdata on which a data pilot has been superimposed.

FIG. 2 is a diagram illustrating the configuration of a transmissionunit in user equipment 200 and the configurations of units forperforming receiving processing and data processing in a base station100. This drawing illustrates a case where an Orthogonal FrequencyDivision Multiplexing (OFDM) system is used.

In a certain piece of terminal 200 (UE#jv), transmission data Data#jv issubjected to inverse fast Fourier transform in an inverse fast Fouriertransformer (IFFT) 220 and is converted into a time-domain transmissionsignals (a1, a2, . . . and, aNf). Subsequently, to the obtainedtime-domain transmission signals (a1, a2, . . . and, aNf) output fromthe IFFT 220, a result of the product of an amplitude adjustment factorPg and a pilot (a pilot of “1” in this example) 235 spread using anorthogonal code Dcode#jv is added for each bit (230). A resultant signalis subjected to high-frequency processing and is transmitted via anantenna. The description of known processing operations such as theaddition of a cyclic prefix, digital-to-analog (D/A) conversion,high-frequency conversion, and amplification is omitted.

The base station 100 receives signals from a plurality of pieces of userequipment 200 at a time via a plurality of antennas under the effect ofthe channel H0 r. The description of known configurations of a RadioFrequency (RF) unit, a quadrature modulation unit, and ananalog-to-digital conversion unit will be omitted.

A pilot header processor 170 in the base station 100 receives a pilotheader and performs despreading of the received signal using a pilotorthogonal code assigned to a corresponding piece of user equipment,thereby distinguishing the piece of user equipment from the other piecesof user equipment and estimating the channel H0 between each of allantennas of the base station and the piece of user equipment. Thechannel H0 is stored in a storage unit 139 in the base station 100.

As described previously, the channel H0 r indicates that it may bechanged from the channel H0 at the time of reception of a pilot header.Subsequently, a data processor 130 performs data decoding processing andchannel estimation processing for each piece of user equipment. In thedrawing, an index “TTL” is an abbreviation for total and represents thecombination of all pieces of user equipment.

In data decoding processing that corresponds to equations (5) and (6)and is illustrated in the upper portion of the block of the base station100 in FIG. 2, first, the data orthogonal code component of all piecesof user equipment is subtracted from the received signal. The dataorthogonal code component of all pieces of user equipment is obtained bymultiplying the amplitude adjustment factor Pg.

Subsequently, in order to correlate a result of the subtraction and thecomplex conjugate transpose H0* of the channel matrix H0 stored in thestorage unit 139, the subtraction result is subjected to multiplication.As a result, received signals are separated on a terminal-by-terminalbasis in accordance with the characteristic of MIMO communication.Subsequently, a fast Fourier transformer (FFT) 135 performs an FFToperation on the obtained signal, so that original transmission data canbe acquired (decoded).

In channel estimation processing illustrated in the lower portion of thedata processor 130 in the base station 100 in FIG. 2, each of signalsreceived from all pieces of user equipment is despread using the codeDcode#jv of a corresponding one of these pieces of user equipment. Usingthe above-described equations (3) and (4), the channel H0 r isestimated. Subsequently, a comparator 137 compares the obtained channelH0 r with the channel H0 stored in the storage unit 139. When thedifference between the estimated values of the channels H0 r and H0 isequal to or larger than a certain value, the channel H0 stored in thestorage unit 139 is updated to the channel H0 r. The estimation of thechannel H0, which is performed by the pilot header processor 170 using apilot header, may be performed at the time of the above-describedinitial setting and the restart of communication. During communication,the update of a channel estimation value is performed using a pilotsuperimposed on a data slot. The channel estimation performed with apilot header is not performed or a result of the channel estimation iseliminated after performance of the channel estimation.

Next, exemplary specific numerical values of main parameters will bedescribed. Both time lengths of a massive MIMO uplink slot and a massiveMIMO downlink slot under consideration are 500 μS. The time length of aheader has not been set yet. Accordingly, an LTE time standard isemployed. The reason for this is that the fifth generation (5G) systemwill be probably compliant with the fourth generation (4G) system. Inthis case, the TDD-LTE standard of 1 slot=500 μS can be employed. Sinceseven OFDM symbols are transmitted, a time length of a single OFDMsymbol becomes approximately 70 μS. When this time length is used for aheader, approximately 32 nS is obtained per OFDM bin (FFT symbol).Accordingly, in a case where one bit is assigned to one bin, a pilotorthogonal code having the length of 2048 bits can be prepared. Thismeans that 2048 pieces of user equipment can be distinguished. MassiveMIMO under consideration estimates the number of pieces of userequipment covered by a single base station at 10 to 20. Accordingly,this code numerical value is sufficiently large.

The data length of a data orthogonal code is 2048×7 since seven OFDMsymbols are included in a single slot. An orthogonal code having thislength can be used. Some methods of using the orthogonal code can beproposed, and will be described below as first and second modifications.

In the first modification in FIG. 3, an exemplary case where seven OFDMsymbols are included in an uplink data slot UL-data and a pilot issuperimposed on only one symbol is illustrated. The other OFDM symbolsare used only for data transmission. In the exemplary case in FIG. 3,although a pilot is superimposed on a first OFDM symbol OFDM#1, a pilotmay be superimposed on another symbol. Alternatively, pilots may besuperimposed on a plurality of OFDM symbols. The length of a single slotis 500 μS. Accordingly, when it can be assumed that there is no channelvariation in this period, a system can be simplified. Thus, an uplinkdata slot can be separated into a plurality of data sections and a pilotcan be superimposed on at least one data section.

The second modification illustrated in FIG. 4 differs from the exemplarycase illustrated in FIG. 3 in that an OFDM frequency component isseparated into a plurality of blocks (four blocks in the drawing) andpilots are superimposed on these blocks. The same orthogonal code isassigned to these blocks. In an exemplary case in FIG. 4, an orthogonalcode having the length of 512 is assigned to each block. Under theassumption that frequency fading occurs, channel estimation can beperformed for each block when there is a frequency characteristic. Thus,a pilot can be superimposed on each of a plurality of blocks into whicha frequency component in an OFDM symbol is separated.

In the embodiment in FIG. 1, exemplary pilot superimposition processingin a time domain is illustrated. When OFDM is employed, frequency domainprocessing can be performed. In this case, since a channel is obtainedfor each OFDM subcarrier, more detailed frequency characteristic can becovered.

A second embodiment in which frequency domain processing is performedwill be described with reference to FIG. 5. This exemplary case issubstantially the same as the exemplary case illustrated in FIG. 1 inwhich time domain processing is performed. The difference between themis that, in a piece of user equipment, the superimposition of a pilot isperformed in a frequency domain before inverse fast Fourier transform(IFFT) is performed and then the conversion into a time-domain signal isperformed by the IFFT 220. In a base station, first, the FET 135converts a received signal into a frequency-domain signal. Subsequently,in a frequency domain, the subtraction of a pilot, correlationprocessing, data demodulation, and channel estimation are performed.

FIG. 6 is a graph illustrating a result of characteristic simulationaccording to an embodiment of the present disclosure. In this example,the number of pieces of user equipment is 3, the number of antennas of abase station is 100, the type of modulation is quadraphase shift keying(QPSK), and an FET size is 512. An amplitude adjustment factor Pg is setto 1.0, and the description of this value will be described later. Sincea high speed is not required in uplink transmission from user equipment,it is considered that QPSK is generally used. FIG. 6 illustrates theeffect of channel estimation performed with a pilot superimposed ondata. A horizontal axis represents a deviation from a channel estimatedwith a pilot header which is caused by Doppler effect, and a verticalaxis represents the change in bit error rate (BER) with the deviation.The term of “w/o correction” indicates that there is no channel update.When a channel error exceeds 30%, BER markedly increases. This indicatesthat the massive MIMO channel diagonalization is inadequate. A case inwhich update is performed with a pilot superimposed on data isrepresented by the term of “w/ correction” in the drawing. In spite ofthe fact that a channel error increases, the BER is maintained at a lowconstant value. The reason why an error-free result is not obtained isthat the interference of other pieces of user equipment cannot becompletely eliminated even if correlation processing is performed.

FIGS. 7A and 7B are diagrams illustrating a situation in which theconstellation of received signals rotated because of Doppler effect isrecovered after a channel estimation update has been performed with apilot superimposed on data. FIG. 7A illustrates the constellation when achannel error is 100%. Referring to the drawing, the constellation isrotated. FIG. 7B illustrates the constellation when the update isperformed. As is apparent from the drawing, the constellation isrecovered.

FIG. 8 illustrates a simulation result of the effect of the amplitudeadjustment factor Pg. This simulation is performed under the sameconditions as those described with reference to FIG. 6. The amplitudeadjustment factor Pg serves as an index for determining when to separatea pilot from data. When the value of this factor is small, a pilot valuebecomes inaccurate because of the interference of data. When the valueof this factor is large, transmission power increases. Referring to thisdrawing, a constant BER value is obtained when Pg>0.3. Since a pilot indata is obtained by despreading a data orthogonal code, the pilot ismultiplied by a factor of the data orthogonal code. In this example,since a code having the length of 512 is used, the pilot is multipliedby 512. As is apparent from this drawing, the interference of datamarkedly decreases after Pg exceeds a certain value.

The present disclosure can also be applied to a communication systemother than the massive MIMO communication system in which channelestimation is required. For example, in the case of a pilot signal to beused in MIMO performed in CDMA, by performing code spreading of achannel estimation signal, data, and a pilot and multiplexing them, theeffect of the present disclosure can be obtained.

According to an embodiment of the present disclosure, by preparingheaders for pilot transmission and headers for the other purposes inuplink and placing the pilot headers at regular intervals, the load ofpilot transmission can be reduced. Furthermore, by multiplexing data anda pilot signal and embedding the pilot signal in data, a base stationcan perform channel estimation at the time of data reception and canalways obtain the latest channel information.

The separation among a plurality of pieces of user equipment isperformed with orthogonal codes. Consequently, signals from terminals inclose vicinity of one another can be separated. Furthermore, sincesignals from the other cells can be separated, pilot contamination canbe solved. In the present disclosure, since a long-period orthogonalcode can be used, a code shortage does not occur.

According to the present disclosure, the following method and thefollowing apparatuses are provided.

-   (1) A MIMO communication method of performing MIMO communication    between a base station having a plurality of antennas and each of a    plurality of terminals covered by the base station using uplink data    slots and downlink data slots that are alternately placed on a time    axis, comprising:

in each of the plurality of terminals,

setting pilot transmission headers to be used for transmission of apilot signal at predetermined header intervals as headers of the uplinkdata slots;

spreading the pilot signal in the pilot transmission headers usingcorresponding one of orthogonal codes that are assigned to the pluralityof terminals and are orthogonal to one another and transmitting thespread pilot signal;

spreading the pilot signal in the uplink data slots using a dataorthogonal code that is different from the orthogonal code used in thepilot transmission headers, superimposing the spread pilot signal ontransmission data, and transmitting the transmission data;

in the base station,

despreading a received signal that is transmitted from each of theplurality of terminals and is included in the pilot transmission headersusing the orthogonal code assigned to the terminal, estimating a channelbetween each of all antennas of the base station and the terminal, andstoring a value of the estimated channel;

demodulating the transmission data transmitted from the terminal on thebasis of the value of the estimated channel;

decoding a received signal included in the uplink data slots using thedata orthogonal code assigned to the terminal and estimating a currentchannel between each of all antennas of the base station and theterminal; and

comparing the stored value of the estimated channel with a value of theestimated current channel and updating the stored value of the estimatedchannel to the value of the estimated current channel when a differencebetween them is larger than a value set in advance.

-   (2) The MIMO communication method according to (1), comprising:

in each of the plurality of terminals,

adjusting an amplitude of the pilot signal;

additively superimposing the amplitude-adjusted pilot signal on thetransmission data in a time direction or a frequency direction;

transmitting a signal obtained from the superimposition;

in the base station,

receiving a signal transmitted from the terminal; and

subtracting the amplitude-adjusted pilot signal from the received signalin the time direction or the frequency direction and performingdemodulation for MIMO communication using a remaining signal.

-   (3) The MIMO communication method according to (1), wherein the    uplink data slot is separated into a plurality of data sections and    the pilot signal is superimposed on at least one of the plurality of    data sections.-   (4) The MIMO communication method according to (3), wherein the data    sections are orthogonal frequency division multiplexing (OFDM)    symbols.-   (5) The MIMO communication method according to (4), wherein a    frequency component in the OFDM symbol is separated into a plurality    of blocks and the pilot signal is superimposed on each of the    blocks.-   (6) The MIMO communication method according to (1),

wherein OFDM is used for data communication,

wherein each of the plurality of terminals performs, in a frequencydomain, the superimposition of the pilot signal on the transmission datain the uplink data slots before performing inverse fast Fouriertransform (IFFT) of OFDM, and

wherein the base station performs fast Fourier transform (FFT) on thereceived signal so as to convert the received signal into afrequency-domain signal and performs the subtraction, the demodulation,and the channel estimation in the frequency domain.

-   (7) A terminal for performing MIMO communication with a base station    apparatus having a plurality of antennas, the terminal comprising:

a transmission unit configured to transmit data to the base stationapparatus using uplink data slots, and

wherein the transmission unit sets pilot transmission headers to be usedfor transmission of a pilot signal at predetermined header intervals asheaders of the uplink data slots,

wherein the transmission unit spreads the pilot signal in the pilottransmission headers using corresponding one of orthogonal codes thatare assigned to a plurality of terminals and are orthogonal to oneanother and transmits the spread pilot signal, and

wherein the transmission unit spreads the pilot signal in the uplinkdata slots using a data orthogonal code that is different from theorthogonal code used in the pilot transmission headers, superimposes thespread pilot signal on transmission data, and transmits the transmissiondata.

-   (8) A base station apparatus for performing MIMO communication with    a plurality of terminals covered by the base station apparatus    comprising:

a pilot transmission header processing unit configured to despread asignal that is received from each of the plurality of terminals via aplurality of antennas and is included in pilot transmission headersusing an orthogonal code assigned to the terminal, estimate a channelbetween each of all of the plurality of antennas of the base station andthe terminal, and store a value of the estimated channel; and

a data processing unit configured to process the signal received fromeach of the plurality of terminals, and

wherein the data processing unit demodulates transmission datatransmitted from the terminal on the basis of the value of the estimatedchannel,

wherein the data processing unit decodes a received signal included inthe uplink data slots using a data orthogonal code assigned to theterminal and estimates a current channel between each of all of theplurality of antennas of the base station and the terminal, and

wherein the data processing unit compares the stored value of theestimated channel with a value of the estimated current channel andupdates the stored value of the estimated channel to the value of theestimated current channel when a difference between the values is largerthan a value set in advance.

Although the preferred embodiments of the present disclosure have beendescribed above, various modifications and various changes other thanthose described above can be made. That is, it is obvious to thoseskilled in the art that various changes, various combinations, otherembodiments can be made in consideration of design or another factorinsofar as they are within the scope of the present disclosure asclaimed or the equivalents thereof.

REFERENCE SIGNS LIST

100: base station

130: data processor

135: FFT

137: comparator

200: user equipment (terminal)

220: IFFT

What is claimed is:
 1. A MIMO communication method of performing MIMOcommunication between a base station having a plurality of antennas andeach of a plurality of terminals covered by the base station usinguplink data slots and downlink data slots that are alternately placed ona time axis, comprising: in each of the plurality of terminals, settingpilot transmission headers to be used for transmission of a pilot signalat predetermined header intervals as headers of the uplink data slots;spreading the pilot signal in the pilot transmission headers usingcorresponding one of orthogonal codes that are assigned to the pluralityof terminals and are orthogonal to one another and transmitting thespread pilot signal; spreading the pilot signal in the uplink data slotsusing a data orthogonal code that is different from the orthogonal codeused in the pilot transmission headers, superimposing the spread pilotsignal on transmission data, and transmitting the transmission data; inthe base station, despreading a received signal that is transmitted fromeach of the plurality of terminals and is included in the pilottransmission headers using the orthogonal code assigned to a respectiveone of the terminals, estimating a channel between each of all antennasof the base station and the respective terminal, and storing a firstvalue of the estimated channel; demodulating the transmission datatransmitted from the respective terminal on a basis of the first valueof the estimated channel; decoding a received signal included in theuplink data slots using the data orthogonal code assigned to therespective terminal and estimating a current channel between each of allantennas of the base station and the respective terminal; and comparingthe stored first value of the estimated channel with a second value ofthe estimated current channel and updating the stored first value of theestimated channel to the second value of the estimated current channelwhen a difference between the values is larger than a third value set inadvance.
 2. The MIMO communication method according to claim 1, furthercomprising: in each of the plurality of terminals, adjusting anamplitude of the pilot signal; additively superimposing theamplitude-adjusted pilot signal on the transmission data in a timedirection or a frequency direction; and transmitting a signal obtainedfrom the superimposition; in the base station, receiving a signaltransmitted from the respective terminal; and subtracting theamplitude-adjusted pilot signal from the received signal in the timedirection or the frequency direction and performing demodulation forMIMO communication using a remaining signal.
 3. The MIMO communicationmethod according to claim 1, wherein the uplink data slot is separatedinto a plurality of data sections and the pilot signal is superimposedon at least one of the plurality of data sections.
 4. The MIMOcommunication method according to claim 3, wherein the data sections areorthogonal frequency division multiplexing (OFDM) symbols.
 5. The MIMOcommunication method according to claim 4, wherein a frequency componentin the OFDM symbol is separated into a plurality of blocks and the pilotsignal is superimposed on each of the plurality of blocks.
 6. The MIMOcommunication method according to claim 1, wherein OFDM is used for datacommunication, wherein each of the plurality of terminals performs, in afrequency domain, the superimposition of the pilot signal on thetransmission data in the uplink data slots before performing inversefast Fourier transform (IFFT) of OFDM, and wherein the base stationperforms fast Fourier transform (FFT) on the received signal so as toconvert the received signal into a frequency-domain signal and performsthe subtraction, the demodulation, and the channel estimation in thefrequency domain.
 7. A terminal for performing MIMO communication with abase station apparatus having a plurality of antennas, the terminalcomprising: a transmitter configured to transmit data to the basestation apparatus using uplink data slots and transmit pilottransmission headers to the base station using time slots separate fromthe uplink data slots; and processing circuitry configured to set thepilot transmission headers to be used for transmission of a pilot signalat predetermined header intervals as headers of the uplink data slots,spread the pilot signal in one of the pilot transmission headers usingcorresponding one of orthogonal codes that are assigned to a pluralityof terminals and are orthogonal to one another and transmit the spreadpilot signal, wherein the pilot signals for all of the plurality ofterminals are spread to overlap in time within a same time slot of thepilot transmission header, wherein the processing circuitry isconfigured to spread the pilot signal in one of the uplink data slotsusing a data orthogonal code that is different from the orthogonal codeused in the pilot transmission headers, and multiplex the spread pilotsignal on transmission data, and the transmitter is configured totransmit the transmission data in the one of the uplink data slots,wherein the transmission data for all of the plurality of terminalsoverlap in time within the one of the uplink data slots.
 8. A basestation apparatus for performing MIMO communication with a plurality ofterminals covered by the base station apparatus comprising: a pluralityof antennas; and processing circuitry configured to despread a signalthat is received from each of the plurality of terminals via theplurality of antennas and is included in pilot transmission headersusing an orthogonal code assigned to a respective one of the terminals,estimate a channel between each of all of the plurality of antennas ofthe base station and the respective terminal, store a first value of theestimated channel, process the signal received from each of theplurality of terminals, demodulate transmission data transmitted fromthe respective terminal on a basis of the first value of the estimatedchannel, decode a received signal included in the uplink data slotsusing a data orthogonal code assigned to the respective terminal andestimate a current channel between each of all of the plurality ofantennas of the base station and the respective terminal, and comparethe stored first value of the estimated channel with a second value ofthe estimated current channel and updates the stored first value of theestimated channel to the second value of the estimated current channelwhen a difference between the values is larger than a third value set inadvance.